Widely-wavelength-tunable brillouin fiber laser with improved optical signal-to-noise ratio based on parity-time symmetric and saturable absorption effect

A widely-wavelength-tunable Brillouin fiber laser(BFL)with improved optical signal-to-noise ratio(OSNR)based on parity-time(PT)symmetric and saturable absorption(SA)effect is present.This novel BFL realizes PT symmetry and SA effect through polarization-maintaining erbium-doped fiber(PM-EDF)Sagnac loop,which is composed of a PM-EDF,a coupler and two polarization controllers(PCs).By using the inherent birefringence characteristic of PM-EDF,two feedback loops in orthogonal polarization state are formed when the Strokes signal in injected.One of these loops provides gain in the clockwise direction with in the Sagnac loop,while the other loop generates loss in the counterclockwise direction.By adjusting the PCs to control the polarization state of the PM-EDF,a single-longitudinal-mode(SLM)BFL can be achieved,as the PT symmetry is broken when the SA participating stimulated Brillouin scattering(SBS)gain and loss are well-matched and the gain surpasses the coupling coefficient.Compared to previous BFLs,the proposed BFL has a more streamlined structure and a wider wavelength tunable range,at the same time,it is not being limited by the bandwidth of the erbium-doped fiber amplifier while still maintaining narrow linewidth SLM output.Additionally,thanks to SA effect of the PM-EDF,the PT symmetric SBS gain contract is enhanced,resulting in a higher optical signal-to-noise(OSNR).The experimental results show that the laser has a wide tunable range of 1526.088 nm to 1565.498 nm,an improved OSNR of 77 dB,and a fine linewidth as small as 140.5 Hz.

Two-hertz-linewidth Nd:YAG lasers at 1064 nm stabilized to vertically mounted ultra-stable cavities

Two Nd：YAG lasers operating at 1064 nm are separately servo-locked to two vertically mounted ultra-stable cavities. The optical heterodyne beat between two cavity-stabilized lasers shows that the linewidth of each laser reaches 2 Hz and the average frequency drift reduces to less than 1 Hz/s.

Waveguide external cavity narrow linewidth semiconductor lasers

Narrow linewidth light source is a prerequisite for high-performance coherent optical communication and sensing.Waveguide-based external cavity narrow linewidth semiconductor lasers(WEC-NLSLs)have become a competitive and attractive candidate for many coherent applications due to their small size,volume,low energy consumption,low cost and the ability to integrate with other optical components.In this paper,we present an overview of WEC-NLSLs from their required technologies to the state-of-the-art progress.Moreover,we highlight the common problems occurring to current WEC-NLSLs and show the possible approaches to resolving the issues.Finally,we present the possible development directions for the next phase and hope this review will be beneficial to the advancements of WEC-NLSLs.

Broad bandwidth interference filter-stabilized external cavity diode laser with narrow linewidth below 100kHz

Interference filter-stabilized external cavity diode lasers （ECDLs） have properties of simple configurations, high sta- bilities, and narrow linewidths. However, the interference filter used in common ECDL designs requires an ultra-narrow bandwidth （about 0.3 nm） to achieve mode selection, that is considerably expensive and not yet available for a wide range of wavelengths. In this paper, a robust ECDL using an available broad bandwidth （about 4 nm） interference filter as the wavelength discriminator is constructed and tested. The ECDL demonstrated a narrow Lorentzian fitted linewidth of 95 kHz and a spectral purity of 2.9 MHz. The long-term frequency stability of the ECDL reaches 5.59 x 10 12.

Observation of linewidth narrowing due to a spontaneously generated coherence effect

We investigate the resonance fluorescence spectrum of an atomic three-level ladder system driven by two laser fields. We show that such a system emulates to a large degree a V-type atom with parallel dipole moments-the latter being a system that exhibits spontaneously generated coherence and can display ultrasharp spectral lines. We find a suitable energy scheme in a SSRb atom and experimentally observe the narrowing of the central peak in a rubidium atomic beam. The corresponding spectrum can convindngiy demonstrate the existence of spontaneously generated coherence.

Cavity linewidth narrowing with dark-state polaritons

We present a quantum-theoretical treatment of cavity linewidth narrowing with intracavity electromagnetically in- duced transparency （EIT）. By means of intracavity EIT, the photons in the cavity are in the form of cavity polaritons： bright-state polariton and dark-state polariton. Strong coupling of the bright-state polariton to the excited state induces an effect known as vacuum Rabi splitting, whereas the dark-state polariton decoupled from the excited state induces a narrow cavity transmission window. Our analysis would provide a quantum theory of linewidth narrowing with a quantum field pulse.

Ultra-high spectral purity laser derived from weak external distributed perturbation

Ultra-high spectral purity lasers are of considerable research interests in numerous fields such as coherent optical communication,microwave photonics,distributed optical fiber sensing,gravitational wave detection,optical clock,and so on.Herein,to deeply purify laser spectrum with compact size under normal condition,we propose a novel and practical idea to effectively suppress the spontaneous radiation of the laser cavity through weak external distributed perturbation.Subsequently,a laser configuration consisting of a main lasing cavity and an external distributed feedback cavity is proposed.The feedback signal with continuous spatio-temporal phase transition controlled by a distributed feedback structure is injected into the main cavity,which can deeply suppress the coupling rate from the spontaneous radiation to the stimulated emission and extremely purify the laser spectrum.Eventually,an ultra-narrow linewidth on-chip laser system with a side mode suppression ratio greater than 80 dB,an output linewidth of 10 Hz,and a relative intensity noise less than-150 dB/Hz is successfully obtained under normal conditions.The proposed concept in this work provides a new perspective for extreme regulation of laser parameters by using weak external distributed perturbation,which can be valid for various gain-type lasers with wide wavelength bands.

Power output correction model of narrow linewidth ring fiber laser filtered with whispering gallery mode resonator

A whispering gallery mode resonator(WGMR)filter can narrow laser linewidth while significantly changing the output power characteristics of fiber laser system.It is found that traditional laser output power model is invalid.We report a correction model of a narrow linewidth fiber laser filtered with a WGMR to analyze its power.We believe that the loss of the laser system and the threshold gain increase caused by the WGMR filter lead to the predominate amplified spontaneous emission during the original laser period.According to that,we assume the correction coefficient is an exponential decay related to the Er-doped fiber length in the large loss situation,and we verify it experimentally.As a result,the correction model is valid for WGMR-filtered fiber laser.

Influences of semiconductor laser on fibre-optic distributed disturbance sensor based on Mach-Zehnder interferometer

This paper investigates the influences of a semiconductor laser with narrow linewidth on a fibre-optic distributed disturbance sensor based on Mach-Zehnder interferometer. It establishes an effective numerical model to describe the noises and linewidth of a semiconductor laser, taking into account their correlations. Simulation shows that frequency noise has great influences on location errors and their relationship is numerically investigated. Accordingly, there is need to determine the linewidth of the laser less than a threshold and obtain the least location errors. Furthermore, experiments are performed by a sensor prototype using three semiconductor lasers with different linewidths, respectively, with polarization maintaining optical fibres and couplers to eliminate the polarization induced noises and fading. The agreement of simulation with experimental results means that the proposed numerical model can make a comprehensive description of the noise behaviour of a semiconductor laser. The conclusion is useful for choosing a laser source for fibre-optic distributed disturbance sensor to achieve optimized location accuracy. What is more, the proposed numerical model can be widely used for analysing influences of semiconductor lasers on other sensing, communication and optical signal processing systems.

Coherence transfer from 1064 nm to 578 nm using an optically referenced frequency comb

A laser at 578 nm is phase-locked to an optical frequency comb（OFC） which is optically referenced to a subhertzlinewidth laser at 1064 nm. Coherence is transferred from 1064 nm to 578 nm via the OFC. By comparing with a cavitystabilized laser at 578 nm, the absolute linewidth of 1.1 Hz and the fractional frequency instability of 1.3 × 10^-15 at an averaging time of 1 s for each laser at 578 nm have been determined, which is limited by the performance of the reference laser for the OFC.

Linewidth narrowing in free-space-running diamond Brillouin lasers

This study analyzes the linewidth narrowing characteristics of free-space-running Brillouin lasers and investigates the approaches to achieve linewidth compression and power enhancement simultaneously.The results show that the Stokes linewidth behavior in a free-space-running Brillouin laser cavity is determined by the phase diffusion of the pump and the technical noise of the system.Experimentally,a Stokes light output with a power of 22.5 W and a linewidth of 3.2 kHz was obtained at a coupling mirror reflectivity of 96%,which is nearly 2.5 times compressed compared with the linewidth of the pump(7.36 kHz).In addition,the theorical analysis shows that at a pump power of 60Wand a coupling mirror reflectivity of 96%,a Stokes output with a linewidth of 1.6 kHz and up to 80%optical conversion efficiency can be achieved by reducing the insertion loss of the intracavity.This study provides a promising technical route to achieve high-power ultra-narrow linewidth special wavelength laser radiations.

Probe gain via four-wave mixing based on spontaneously generated coherence

We have studied the probe gain via a double-Λ atomic system with a pair of closely lying lower levels in the presence of two probe and two coherent pump fields. The inversionless gain can be realized by using nondegenerate four-wave mixing under the condition of spontaneously generated coherence（SGC） owing to near-degenerate lower levels. Note that by using SGC, two probe fields can be amplified with more remarkable amplitudes, and the gain spectra of an extremely narrow linewidth can be obtained. Last but not least, our results show that the probe gain is quite sensitive to relative phases due to the SGC presence which allows one to modulate the gain spectra periodically by phase modulation, and can also be influenced by all laser field intensities and frequencies, and the angles between dipole elements.

Experiments on Nanosecond Optical Parametric Generation for 486.0 nm in BBO Crystal

In this paper, an injection-seeded nanosecond optical parametric generation (OPG) using BBO crystal, which combines relatively low thresholds with a simple and compact configuration, was demonstrated. By seeding externally with distributed feedback (DFB) diode laser at 1313 nm wavelength, pumped by 355 nm laser pulse, the maximum blue laser output power of 1.36 W at a rate of 100 Hz and with linewidth less than 0.13 nm were obtained, and the maximum optical to optical conversion efficiency was to 21.2%.

Six kilowatt record all-fiberized and narrow-linewidth fiber amplifier with near-diffraction-limited beam quality

In this work,an all-fiberized and narrow-linewidth fiber amplifier with record output power and near-diffraction-limited beam quality is presented.Up to 6.12 kW fiber laser with the conversion efficiency of approximately 78.8%is achieved through the fiber amplifier based on a conventional step-index active fiber.At the maximum output power,the 3 dB spectral linewidth is approximately 0.86 nm and the beam quality factor is M_(x)^(2)=1.43,M_(y)^(2)=1.36.We have also measured and compared the output properties of the fiber amplifier employing different pumping schemes.Notably,the practical power limit of the fiber amplifier could be estimated through the maximum output powers of the fiber amplifier employing unidirectional pumping schemes.Overall,this work could provide a good reference for the optimal design and potential exploration of high-power narrow-linewidth fiber laser systems.

kW-level,narrow-linewidth linearly polarized fiber laser with excellent beam quality through compact one-stage amplification scheme

In this manuscript, we demonstrate high-power, narrow-linewidth linearly polarized fiber laser with excellent beam quality through compact one-stage amplification scheme. By employing a single-mode–multimode–single-mode structure seed laser, a linearly polarized Yb-doped fiber laser with narrow linewidth and high output power is achieved.This laser, when used as a master oscillator, can be capable of suppressing the ASE in the process of power amplification.Thus, only one-stage amplification structure is used to scale up the laser power, and linearly polarized output with a polarization extinction ration of 14 d B, a narrow linewidth of 0.3 nm and an output power of 1018 W are achieved.Moreover, due to the good beam quality of seed laser and the well-designed amplifier stage, the beam quality of the output laser is near-diffraction-limited with M_x^2 ~1.18 and M_y^2 ~ 1.24 at the maximum power, and without mode instability occurring.

Narrow-linewidth BaGa_(4)Se_(7) optical parametric oscillator

The linewidth of the BaGa_(4)Se_(7)(BGSe)optical parametric oscillator(OPO)was narrowed for the first time,to the best of our knowledge,by inserting a Fabry-Perot(FP)etalon into an L-shaped cavity.When a 15 mm long BGSe(56.3°,0°)was pumped by a 1064 nm laser,the peak wavelength was〜3529 nm,and the linewidth was 4.53 nm[3.64 cm^(-1)]under type I phase matching.After inserting a 350μm thick FP etalon,the linewidth was decreased to 1.27-2.05 nm.When the tilt angle of the etalon was 2.34°,the linewidth was 2.05 nm[1.65 cm^(-1)],and the peak wavelength was still〜3529 nm.When the tilt angle of the etalon was 3.90°,the peak wavelength was 3534.9 nm,and the linewidth was 1.27 nm(1.02 cm^(-1)),which was the narrowest linewidth of a BGSe OPO,to the best of our knowledge.The beam quality was also improved after inserting the FP etalon.

High-power narrow-linewidth diode laser pump source based on high-efficiency external cavity feedback technology

In this research,the highly efficient external cavity feedback technology based on volume Bragg grating(VBG)is studied.By using the structure of a fast axis collimating lens,the beam transformation system,a slow axis collimating lens,and VBG,the divergence angle of the fast and slow axes of the diode laser incident on the VBG is reduced effectively,and the feedback efficiency of the external cavity is improved.Combined with beam combining technology,fiber coupling technology,and precision temperature control technology,a high-power and narrow-linewidth diode laser pump source of kilowatt class is realized for alkali metal vapor laser pumping.The core diameter of the optical fiber is 1000μm,the numerical aperture is 0.22,the output power from the fiber is 1013 W,the fiber coupling efficiency exceeds 89%,and the external cavity efficiency exceeds 91%.The central wavelength is 852.052 nm(in air),which is tunable from 851.956 nm to 852.152 nm,and the spectral linewidth is 0.167 nm.Research results can be used for cesium alkali metal vapor laser pumping.

An ultra-narrow linewidth solution-processed organic laser

Optically pumped lasers based on solution-processed thin-film gain media have recently emerged as low-cost,broadly tunable,and versatile active photonics components that can fit any substrate and are useful for,e.g.,chemo-or biosensing or visible spectroscopy.Although single-mode operation has been demonstrated in various resonator architectures with a large variety of gain media-including dye-doped polymers,organic semiconductors,and,more recently,hybrid perovskites-the reported linewidths are typically on the order of a fraction of a nanometer or broader,i.e.,the coherence lengths are no longer than a few millimeters,which does not enable high-resolution spectroscopy or coherent sensing.The linewidth is fundamentally constrained by the short photon cavity lifetime in the standard resonator geometries.We demonstrate here a novel structure for an organic thin-film solid-state laser that is based on a vertical external cavity,wherein a holographic volume Bragg grating ensures both spectral selection and output coupling in an otherwise very compact(,cm3)design.Under short-pulse(0.4 ns)pumping,Fourier-transform-limited laser pulses are obtained,with a full width at half-maximum linewidth of 900 MHz(1.25 pm).Using 20-ns-long pump pulses,the linewidth can be further reduced to 200 MHz(0.26 pm),which is four times above the Fourier limit and corresponds to an unprecedented coherence length of 1m.The concept is potentially transferrable to any type of thin-film laser and can be ultimately made tunable;it also represents a very compact alternative to bulky grating systems in dye lasers.

Towards tapered-fiber-based all-fiberized high power narrow linewidth fiber laser

Yb-doped fiber laser(YDFL) is a promising kind of laser source due to high efficiency, excellent beam quality, robust configuration, diverse applications and so on. The progress of large-mode-area double cladding active fiber and high power laser diodes have been continuously promoting the power scaling of the YDFLs. However, the further power scaling is blocked by some physic issues including nonlinear effect, thermal-induced mode instability(MI), optical damage and so forth. So, newlydesigned fibers with special structures are invented to address or relieve these limitations. The tapered fiber, which has changed mode area along its longitudinal direction, has been demonstrated to benefit suppressing the nonlinear effect, especially the stimulated Brillouin scattering(SBS) effect. In this paper, we will review the previous work done to explore the advantages of the tapered fiber for high power output, and then for the first time to our knowledge, demonstrate an all-fiberized high power narrow linewidth fiber laser based on a homemade tapered Yb-doped fiber(T-YDF). As a result, an output power of 260 W with a narrow linewidth of ~2 GHz is obtained, with SBS effectively suppressed owing to gradually increased mode area of the T-YDF as well as continuously changed Brillouin frequency shift dependent on the core diameter. Further power scaling is limited by MI, and the features of MI are studied in detail. It is pointed out that, the MI seems the primary and durative limitation factor of the tapered fiber amplifier for high power narrow linewidth output due to the gradually increased effective numerical aperture(NA) and the excitation of the high-order modes, even although it has the advantage to reduce other nonlinear effects. At last,worthwhile discussion about the optimization of the T-YDF and the whole fiber laser system for both SBS and MI suppression is conducted based on the previous and our results. It is concluded that the small diameter at the small-diameter end and low optical NA are preferred in the design of a T-YDF to ensure good beam quality and improve the MI threshold. The good splicing quality and proper coiling or bend state are also necessary.

10 watt-level tunable narrow linewidth 1.5μm all-fiber amplifier

We report here a high-power, wavelength tunable and narrow linewidth 1.5 μm all-fiber laser amplifier based on a tunable diode laser and Er-Yb co-doped fibers. The laser wavelength can be precisely tuned from 1535 nm to 1580 nm, which covers many absorption lines of mid-infrared laser gases, such as C2 H2, HCN, CO, and HI. The maximum laser power is >11 W, and the linewidth is about 200–300 MHz, which is close to the absorption linewidth of the above-mentioned gases. This work provides a suitable pump source for high-power wavelength tunable mid-infrared fiber gas lasers based on low-loss hollow-core fibers.